This invention relates to cooled photodetector apparatus. In particular, the present invention is an improved method for fabricating lead throughs for a Dewar type flask.
Infrared photodetectors have to be operated at low temperatures in order to obtain improved detector performance. The detectors are typically mounted in a double flask of the Dewar type. A Dewar flask consists of an inner flask or "bore" and an outer flask. The infrared detectors are mounted in thermal contact with the top surface of the inner flask, which is cryogenically cooled. Since the detectors are in thermal contact with the inner flask, the detectors operate at the cryogenic temperature.
Because the detectors are typically operated at low temperatures and in a vacuum, electrical connection between each of the detectors in the evacuated portion of the Dewar and the outside Dewar is rather complex. The fabrication techniques used to mount detectors in a Dewar and to provide electrical connection from the detectors to the outside of the Dewar typically involve a large number of hand or manual fabrication steps. These manual fabrication steps are time consuming, tedious, expensive, and generally unsatisfactory.
One typical method of forming electrical conductors in a Dewar is to form electrodes by embedding wires in glass cylinders or tubes and then sealing the tubes to the side of the inner flask. A reflective film is then typically deposited over the inner surfaces and the tubes to reduce heat load in the flask.
The next step in this prior art technique is to attach the photodetectors to a circuit board, which is, in turn, attached to the top of the inner flask. Electrical connections are then made from each of the wires which are embedded in the glass tubes to an electrode on the circuit board. This electrical connection is usually made by a thin diameter wire. Connection is then made to the detector by bonding an additional wire, usually gold, to a metal film on the detector and attaching the opposite end to the PC board electrode.
This prior art technique has a number of disadvantages. First, each of the embedded wires must be inserted in a glass tube and each glass tube must then individually be attached to the side of the inner flask. This process is generally done by hand and is quite time-consuming.
Second, the technique generally requires a circuit board mounted on the top of the inner flask. This results in additional fabrication steps and additional elements in the Dewar which must be cooled.
Third, the outside surface of the inner flask becomes irregular because of the attached tubes. It is not possible, therefore, to conveniently attach a cold shield directly to the outside diameter of the inner flask. Instead, the cold shield must be attached to and/or around the circuit board.
Fourth, the bonding of individual wires between each embedded wire and a detector involves time-consuming manual operations. In addition, the technique requires making a bond to a thin metal film directly on the detector in order to attach the thin wire. The bonding process can cause some heating of the detector, which can adversely affect the detector's performance.
Other techniques for forming electrical conductors on the side of the inner flask have been suggested. For example, silver stripes have been hand painted on the side and top of the inner flask to provide electrical conductive paths. This technique, however, is very unsatisfactory when a large number of detectors are to be mounted in one Dewar.
Another technique is to apply silver decals on the side of the inner flask. This technique also is unsatisfactory because it requires manual dexterity to make connections to the stripes.
Still another technique was suggested by J. J. Long et al in U.S. Pat. No. 3,719,990. In this technique, silver is deposited on the side of the inner flask. The silver is then indexed and scribed, and finally etched to define silver stripes extending on the side of the surface. While this technique eliminates some of the manual operations required by other prior art techniques, it does not eliminate the need for interconnection wires between each of the silver stripes on the side of the flask and each detector.